The present invention relates to a focus detection device in which an image of a target object formed by an objective lens is re-formed, by first and second image forming lenses disposed symmetrically with respect to the optical axis of the objective lens, into first and second optical images on first and second detection means for detecting the first and second optical images, respectively a distance between the first and second optical images is calculated on the basis of illuminance distributions of the first and second optical images detected by the first and second detection means, respectively such that focus condition of the objective lens relative to the target object, and especially the amount of deviation (defocus amount) of the image of the target object from a predetermined image forming plane is detected from the distance between the first and second optical images.
Conventionally, with focus detection devices of this kind, since the first and second optical images are to be formed by light fluxes having passed through the exit pupil of the objective lens and the first and second image forming lenses, it has been a general practice to provide a condenser lens adjacent to the predetermined image forming plane of the objective lens and forwardly of the first and second image forming lenses such that effective apertures of the first and second image forming lenses are projected within the exit pupil of the objective lens. FIGS. 1 and 2 show an optical system of a prior art focus detection device of this kind and formation of images therein, respectively. The known optical system includes an objective lens 2, a predetermined focal plane 4 positioned rearwardly of the objective lens 2 and a condenser lens 6 positioned rearwardly of the predetermined focal plane 4. The condenser lens 6 is constituted by a spherical lens. Furthermore, the known optical system includes a pair of image forming lenses 8 and 10 positioned rearwardly of the condenser lens 6 and a pair of line sensors 12 and 14 positioned at image forming planes of the image forming lenses 8 and 10, respectively. Each of the line sensors 12 and 14 has a charge coupled device (CCD) used as a photo-sensor array. First and second images of a target object are, respectively, formed on the line sensors 12 and 14. As shown in FIG. 2, these images come close to an optical axis 18 in a front focus condition in which an image of a target object to be focused is formed forwardly of the predetermined focal plane 4. On the contrary, the images are spaced away from the optical axis 18 in a rear focus condition in which the image of the target object is formed rearwardly of the predetermined focal plane 4. In an in-focus condition in which the image of the target object is formed on the predetermined focal plane 4, a distance between corresponding points of the two images is set to a specific distance determined by design conditions of the optical system. Therefore, in principle, the focus condition of the optical system can be detected by measuring the distance between the two images.
However, it should be noted here that wave front aberration, especially distortion produced due to deviation of the optical axis of the first and second image forming lenses 8 and 10 from the the optical axis of the condenser lens 6, i.e. the optical axis 18 of the objective lens 2 acting as the primary optical axis of the focus detection optical system exerts different influences on corresponding portions of the first and second images symmetrically with respect to the optical axis 18. Thus, it is difficult to accurately detect the distance between the first and second images.
More specifically, FIG. 3a shows illuminance distribution of the first and second images on the line sensors 12 and 14 in the case where a dark slit image is formed, on the predetermined image forming plane 4, in alignment with the primary optical axis 18. On the other hand, FIGS. 3b and 3c show illuminance distribution of the first and second images in the case where a dark slit image is formed, on the predetermined image forming plane 4, at positions spaced a distance .DELTA.D upwardly and downwardly from the optical axis 18, respectively. In FIGS. 3a to 3c, a distance between the first and second images is represented by D1, D2 and D3, respectively. Essentially, the distances D1, D2 and D3 should be identical with each other. However, as a matter of fact, the distance D1 becomes larger than the distances D2 and D3 equal to each other by the above described influence of wave front aberration, especially distortion produced due to deviation of the optical axis of the first and second image forming lenses 8 and 10 from the optical axis of the condenser lens 6. Furthermore, if the distance .DELTA.D is increased, the distances D2 and D3 are reduced accordingly as shown by the solid line A in FIG. 4. Namely, even when the dark slit image is likewise formed on the predetermined image forming plane 4 as in the case of FIGS. 3a to 3c, the distance between the first and second images decreases gradually as the dark slit image formed on the predetermined image forming plane 4 is spaced further from the primary optical axis 18. This means that in the case where focus detection of the actual image of the target object is performed, the distance between the first and second images may vary at corresponding portions of the first and second images so as to assume different values even in an identical focus condition of the objective lens relative to the target object. Therefore, results of detection of focus condition of the objective lens relative to the target object are subject to change according to positions of main portions of the first and second images.
Japanese Patent Laid-Open Publication No. 32012/1985 has proposed an arrangement which is designed to optically restrain the above described influence of wave front aberration, especially distortion as shown by the broken line B in FIG. 4 by forming the condenser lens into an aspherical shape. However, this known arrangement has such a drawback that production cost of the aspherical condenser lens is high due to the need for manufacture of dies for the condenser lens. Furthermore, the known arrangement is disadvantageous in that sophisticated design of the aspherical lens is required to be performed in order to effectively restrain the influence of wave front aberration, thereby resulting in an expensive focus detection device.